Our goal in this paper is to analyze the physics that could arise from large neutrino magnetic moments as suggested by XENON1T, rather than assessing the possibility of moments larger than estimated theoretical bounds [1]}, [2]}, [3]}. We estimate the consequences of such moments  on the time
evolution of the helicity of primordial neutrinos, and defer discussion of the possibly smaller effect of gravitational fields to later [4]}.
While the precise magnitude of the helicity rotation depends on the magnetic moment in addition to the not-well known cosmic and galactic magnetic field structures, we examine the feasibility
of observing helicity flips for primordial neutrinos and
antineutrinos. We find that even a moment several orders of magnitude smaller than \(\mu _{1T}\)  could lead to significant helicity changes of neutrinos as they propagate through the cosmos as well as the Milky Way. As we discuss, the detection
rates for primordial neutrinos are sensitive to both
their helicity structure as well as whether they are Dirac or
Majorana fermions. Dirac neutrinos are potential probes of cosmic and galactic magnetic fields together with density fluctuations in the early universes, independent of the cosmic microwave background. We assume \(\hbar =c=1\)  throughout.
